Self-Controlled Cleaving Method for Silicon DRIE Process Cross-Section Characterization

At a Glance

Metadata	Details
Publication Date	2021-05-08
Journal	Micromachines
Authors	Dmitry A. Baklykov, Mihail Andronic, Olga S. Sorokina, Sergey S. Avdeev, Kirill A. Buzaverov
Institutions	Institute of Theoretical and Applied Electrodynamics, Bauman Moscow State Technical University
Citations	16
Analysis	Full AI Review Included

Executive Summary

This research addresses critical metrology challenges in characterizing high-aspect ratio (AR) silicon microstructures fabricated using the Bosch Deep Reactive Ion Etching (DRIE) process.

Core Problem: Traditional cleaving methods (diamond scribing) or intersecting auxiliary lines introduce significant physical damage and etching defects, leading to highly inaccurate cross-section profile measurements.
Proposed Solution: A highly controllable, defect-free cleaving method utilizing etched dashed auxiliary lines incorporating sharp stress concentrators.
Key Achievement: The dashed line method successfully controls substrate cleavage without intersecting the target microstructures, thereby eliminating metrology errors caused by localized etching defects.
Defect Analysis: Direct transverse crossing auxiliary lines were shown to cause severe profile narrowing (silicon “build-up” defects) due to increased polymerization at the intersection corners.
Quantified Error: Profile narrowing reached up to 85% for sub-20 µm trenches when using transverse crossing lines, compared to a maximum deviation of less than 12% when using the non-intersecting dashed line reference method.
Process Context: The study focused on optimizing the Bosch process for structures ranging from 2 to 50 µm in width, achieving aspect ratios greater than 10 (up to 50).

Technical Specifications

Parameter	Value	Unit	Context
Substrate Material	p-type Silicon, <100>	10-20 Ω·cm	Diced 25 x 25 mm² samples
Hard Mask Material	Thermal Silicon Dioxide (SiO₂)	4 µm	Protective layer for DRIE
Target Line Width (W) Range	2 to 50	µm	Tested microstructures
Target Line Length (L)	1000	µm	Standard test structure length
Aspect Ratio (W:D) Tested	1:1 and 1:10	N/A	Density ratios
Maximum Aspect Ratio Achieved	> 50	N/A	For 50 µm lines (W:D = 1:1)
Etching Depth (50 µm lines)	> 500	µm	All experiments
DRIE Operating Temperature	5	°C	Bosch process cooling
ICP Power (Passivation/Etching)	1200-1500	W	Inductively Coupled Plasma
RF Power (Breakthrough)	50	W	Radiofrequency power (Highest)
Pressure (Etching Step)	40	mTorr	C₄F₈/SF₆/O₂ mixture
Profile Narrowing (Transverse Lines)	Up to 85	%	Observed for sub-20 µm trenches
Profile Narrowing (Dashed Lines)	< 12	%	Maximum deviation from original width
Etching Rate (Reference, 50 µm, 1:1)	0.527	µm/cycle	Highest rate observed
Selectivity (Reference, 50 µm, 1:1)	243	N/A	Ratio of Si etch rate to SiO₂ etch rate

Key Methodologies

Substrate Preparation and Masking: 4 µm thermal SiO₂ was used as the hard mask on p-type silicon substrates. Pattern transfer was achieved using 4 µm SPR220 photoresist and laser lithography, followed by RIE using CHF₃/Ar gases to etch the SiO₂ mask.
Deep Reactive Ion Etching (DRIE): A three-stage Bosch process was employed at 5 °C, utilizing C₄F₈ (passivation) and SF₆/O₂ (etching/breakthrough). The breakthrough step used 50 W RF power to efficiently remove polymer from the trench bottoms.
Test Topology Design: Target microstructures (lines 2-50 µm wide) were designed with two density ratios (W:D = 1:1 and 1:10) to study Aspect Ratio Dependent Etching (ARDE) and microloading effects.
Cleaving Method Comparison:
- Transverse Crossing Lines: Auxiliary lines (S = 5, 50, 100 µm) were etched directly across the target lines to force cleavage. This method was analyzed for resulting silicon “build-up” defects (polymerization).
- Dashed Auxiliary Lines (Reference): Non-intersecting auxiliary lines (S = 20 µm) were etched adjacent to the target structures. These lines incorporated sharp angular stress concentrators (providing up to 5 times higher maximum stress than nominal) to ensure controlled, defect-free cleavage.
Metrology: Cross-section profiles were analyzed using Field Emission Scanning Electron Microscopy (FE-SEM) to quantify etching rate (V, µm/cycle), selectivity, and profile angle (A, degree) for all tested cleaving methods and structure geometries.

Commercial Applications

The self-controlled cleaving method is critical for reliable metrology in industries requiring high-precision, high-aspect ratio silicon structures:

Micro-Electro-Mechanical Systems (MEMS): Essential for optimizing the Bosch process parameters (profile angle, uniformity) during the fabrication of highly sensitive devices like accelerometers, gyroscopes, and pressure sensors.
Advanced Semiconductor Packaging: Used for quality control in the manufacturing of Through-Silicon Vias (TSV) and interposers, where sidewall profile integrity is paramount for electrical isolation and reliability.
Integrated Optoelectronics: Applicable in the fabrication of silicon photonics components, including waveguides and optical gratings, where precise control over sidewall roughness and profile angle is necessary to minimize light scattering losses.
Microfluidics: Enables accurate characterization of deep, narrow channels and micro-pumps used in lab-on-chip systems, ensuring reproducible fluidic performance.
High-Density Storage and Sensing: Relevant for creating high-aspect ratio pillars or trenches used in advanced memory architectures or high-surface-area chemical sensors.

View Original Abstract

Advanced microsystems widely used in integrated optoelectronic devices, energy harvesting components, and microfluidic lab-on-chips require high-aspect silicon microstructures with a precisely controlled profile. Such microstructures can be fabricated using the Bosch process, which is a key process for the mass production of micro-electro-mechanical systems (MEMS) devices. One can measure the etching profile at a cross-section to characterize the Bosch process quality by cleaving the substrate into two pieces. However, the cleaving process of several neighboring deeply etched microstructures is a very challenging and uncontrollable task. The cleaving method affects both the cleaving efficiency and the metrology quality of the resulting etched microstructures. The standard cleaving technique using a diamond scriber does not solve this issue. Herein, we suggest a highly controllable cross-section cleaving method, which minimizes the effect on the resulting deep etching profile. We experimentally compare two cleaving methods based on various auxiliary microstructures: (1) etched transverse auxiliary lines of various widths (from 5 to 100 μm) and positions; and (2) etched dashed auxiliary lines. The interplay between the auxiliary lines and the etching process is analyzed for dense periodic and isolated trenches sized from 2 to 50 μm with an aspect ratio of more than 10. We experimentally showed that an incorrect choice of auxiliary line parameters leads to silicon “build-up” defects at target microstructures intersections, which significantly affects the cross-section profile metrology. Finally, we suggest a highly controllable defect-free cross-section cleaving method utilizing dashed auxiliary lines with the stress concentrators.

Tech Support

Original Source

DOI: https://doi.org/10.3390/mi12050534

References

2003 - Critical tasks in high aspect ratio silicon dry etching for microelectromechanical systems [Crossref]
2019 - MEMS at Bosch-Si plasma etch success story, history, applications, and products [Crossref]
2018 - DREM2: A facile fabrication strategy for freestanding three dimensional silicon micro-and nanostructures by a modified Bosch etch process [Crossref]
2014 - Deep silicon etching: Current capabilities and future directions
2005 - Advanced etching of silicon based on deep reactive ion etching for silicon high aspect ratio microstructures and three-dimensional micro-and nanostructures [Crossref]